This invention is related to hard, wear resistant coatings vapour deposited over a metallic or non-metallic surface, in particular to tools utilized in industrial, medical and dental cutting, and form scraping.
Hard wearing surfaces are in common use in various industries, and such hard wearing surfaces are frequently obtained by coating the surface of a tool made of steel or similar metal, or other hard, enduring material, with a layer of hard wearing ceramic substance, such as carbides, nitrides and carbonitrides, or providing a hard microcrystalline diamond coating. There are known methods for obtaining hard wearing coatings, such as for example, having a coating of diamond particles in combination with a carbide or nitride layer and then filling the gaps between the abrasive particles with a softer intermetallic compound. Another known method is vapour deposition of hard-wearing ceramic materials from plasma or by utilising molten ceramic substances. Hard wearing surfaces for use on medical, surgical and dental tools have additional requirements, as such surgical and dental tools need to be frequently sterilised, hence medical tools have to be corrosion resistant. A device for yielding hard ceramic surfaces by cathodic arc plasma deposition is described in U.S. Pat. No. 4,851,095, issued to M. A. Scobey et al. on Jul. 25, 1989. The apparatus of Scobey et al. utilises a high intensity ion flux. Vapour deposition of a hard ceramic material, such as titanium or zirconium nitride on a stainless steel or titanium surface by utilizing a molten evaporant and a hollow cathode, is described in U.S. Pat. No. 5,152,774, issued to W. A. Schroeder on Oct. 6, 1992. The vapour deposition of Schroeder is conducted at relatively low temperature, thus the substrate will have lost little of its initial high strength properties, however, the requirement of low surface roughness of the deposited layer is not addressed by U.S. Pat. No. 5,152,774. In U.S. Pat. No. 4,981,756, issued to H. S. Rhandhawa on Jan. 1, 1991, a method is taught to coat surgical tools and instruments by cathodic arc plasma deposition. The ceramic coating obtained by this technology is a nitride, carbide or carbonitride of zirconium or hafnium, in a single layer of 3-10 xcexcm thickness. U.S. Pat. No. 4,981,756 also refers to various publications describing known equipment for obtaining hard-wearing surfaces by cathodic arc plasma deposition. U.S. Pat. Nos. 5,940,975 and 5,992,268 issued to T. G. Decker et al. on Aug. 24, 1999 and Nov. 30, 1999, respectively, teach hard, amorphous diamond coatings obtained in a single layer on thin metallic blades or similar metallic strips utilizing filtered cathodic arc plasma generated by vaporizing graphite. It is noted that no interlayer is formed between the blade surface and the deposited amorphous diamond coating.
It is known to have titanium and titanium nitride coated dental tools and surgical instruments wherein the coating is obtained by conventional cathodic arc deposition applied to corrosion resistant stainless steel substrates. The cutting surfaces of such medical tools need to be smooth, as well as hard-wearing to prevent trapping and retaining materials which can be harmful to the patient. Hence, another requirement is that the cutting edges be very straight, sharp and nick-free to avoid damage to the surrounding flesh and skin during dental treatment. There are known methods described, wherein the cutting tips of surgical instruments made of steel have been sand-blasted and then coated with a hard-wearing ceramic composition, however this method may, or is likely to increase surface roughness and unevenness, rather than eliminate it. The grain size of deposits obtained in conventional cathodic plasma arc methods may range between 0.5 to 10 xcexcm. Any post-deposition heat treatment which may be required to maintain corrosion resistance of the substrate, may lead to internal stresses in the coating due to differences in the grain size, and can eventually lead to abrasion, spalling, crack formation, grain separation, surface fractures, uneven edges and rough surfaces, and such like, which can drastically reduce the wear resistance and durability of surgical instruments and dental tools. None of the above discussed methods are concerned with even grain size and surface structure, and low micro-roughness of the vapour deposited hard, ceramic coatings, which have particular importance for dental and surgical tools, and in other applications where straight, sharp, even and nick-free edges are essential requirements.
There is a need for a method to obtain fine grained, hard wearing ceramic surfaces having low micro-roughness, sharp even edges, which can also withstand post-deposition heat treatment without detriment and degradation of the coating.
An object of the invention is to obtain a coating made of alternating metal and metal ceramic layers of relatively even surface structure and grain size over a requisite surface area of a hard substrate. The coating is obtained by first cleaning, then optionally ion nitriding the surface of a steel, titanium, carbide or similar hard substrate, and subsequently vapour depositing in a cathodic arc plasma deposition device alternating metal and ceramic layers utilizing a magnetically filtered cathodic arc plasma. The magnetic filtration regulates the evenness of the grain size of the deposited layer, and thus a hard-wearing surface having low micro-roughness can be obtained.
According to one embodiment of the present invention a wear resistant, composite vapour deposited metal ceramic coating is provided on a substrate capable of electrical conduction. The coating includes at least one metallic layer selected from the group consisting of titanium, chromium, vanadium, aluminum, molybdenum, niobium, tungsten, hafnium, zirconium and alloys thereof and having a metallic layer thickness. The coating further includes at least one ceramic layer selected from the group consisting of nitrides, carbides, carbohydrides, oxycarbides and oxynitrides. The composite, vapour deposited metal-ceramic coating has a thickness greater than not 0.25 xcexcm, a micro-roughness less than the total thickness of the uppermost ceramic layer, and a micro-hardness in excess of 20 GPa.
The substrate may be of steel. The steel may have an ion nitrided surface layer between it and the composite vapour deposited metal-ceramic coating.
The composite vapour deposited coating may have at least one pair of a metal layer and a ceramic layer having a common metal ion component.
The vapour deposited coating may comprise a multiplicity of pairs of metal and ceramic layers having a common metal ion component.
The composite vapour deposited metal-ceramic coating may be heat treated subsequent to deposition.
The coating thickness may range between 0.25 xcexcm and 20 xcexcm.
The thickness of the metal layer may range between 0.05 xcexcm and 0.5 xcexcm.
The vapour deposited metal-ceramic coating may comprise a portion of a surface of a dental tool, a surgical tool or a cutting tool.
A process is provided for producing a wear resistant, composite vapour deposited metal-ceramic coating on the surface of the substrate capable of electrical conduction, the process comprise of the steps of:
i) providing a substrate capable of electrical conduction, having a surface and cleaning said surface by applying at least one cleaning method selected from the group consisting of chemical cleaning, electrolytic cleaning, grinding, polishing, and ion bombardment to produce a cleaned substrate;
ii) placing said cleaned substrate in the vacuum chamber of a vapour depositing device capable of providing controlled electric and magnetic fields and having a substrate holder capable of holding at least one substrate, a target electrode holder and an inlet for a vapour depositing atmosphere of controlled composition and pressure;
iii) providing a target electrode within said vacuum chamber, of one of the metals selected from the group consisting of titanium, chromium, vanadium, aluminum, molybdenum, niobium, tungsten, hafnium, zirconium, and alloys thereof;
iv) providing a vapour depositing atmosphere within said vacuum chamber, comprising at least one of the gases selected from the group consisting of argon, nitrogen, methane or other hydrocarbon gas and oxygen.
v) optionally treating said surface of said substrate in an ion nitriding process step;
vi) applying electric potential and a filtering magnetic field in an argon atmosphere within said vacuum chamber, to obtain a first, vapour deposited metal layer selected from the group consisting of titanium, chromium, vanadium, aluminum, molybdenum, niobium, tungsten, hafnium, zirconium, and alloys thereof, on said surface of said substrate;
vii) applying electric potential and a filtering magnetic field in an atmosphere within said vacuum chamber, containing at least one of the gases selected from the group consisting of nitrogen, methane, oxygen and water vapour, to obtain a second, vapour deposited layer of a ceramic compound of a metal selected from the group consisting of titanium, chromium, vanadium, aluminum, molybdenum, niobium, tungsten, hafnium, zirconium, and alloys thereof, on said first layer deposited on said surface of said substrate;
viii) repeating steps vi) and vii), thereby obtaining a third, vapour deposited metal layer and a fourth, vapour deposited ceramic compound layer on said surface of said substrate;
ix) removing said substrate having at least four vapour deposited layers on said substrate surface, from said vapour depositing device; and
x) heat treating the obtained vapour deposited coating comprising at least four vapour deposited layers on said substrate surface.
Steps vi) and vii) may be repeated to provide a fifth, vapour deposited metal layer and a sixth vapour deposited ceramic compound layer on the surface of the substrate prior to the heat treatment.
Steps vi) and vii) may be repeated to provide a first multiplicity of vapour deposited metal layers and a second multiplicity of vapour deposited ceramic compound layers on the surface of the substrate prior to the heat treatment.
The substrate may be of steal.